1. Field of the Invention
The invention relates to a position setting method of a photosensitive body for suppressing friction between a belt and the photosensitive body as an image holding body and to an image forming apparatus.
2. Related Background Art
Ordinarily, in a multicolor image forming apparatus such as a color electrophotographic printer or the like, for example, image holding bodies (hereinbelow, referred to as photosensitive drums) for forming toner images of K (black), Y (yellow), M (magenta), and C (cyan) are arranged in a line on a belt at predetermined intervals. In the following description, for example, it is assumed that the photosensitive drums are arranged from an upstream in the conveying direction of a sheet toward its downstream in order of K, Y, M, and C.
In order to simultaneously start the rotation of those four photosensitive drums, simultaneously stop the rotation of them, and laminate the toner images of four colors of K, Y, M, and C to the same position on the sheet conveyed on the belt, assuming that time when an image is irradiated onto the photosensitive drum K is set to reference time Tk, time Ty when an image is irradiated onto the photosensitive drum Y has to be equal to time after the elapse of time Lky/V obtained by dividing a distance Lky from a contact point of the photosensitive drum K and the belt to a contact point of the photosensitive drum Y and the belt by a conveying speed V of the belt.
Time Tm when an image is irradiated onto the photosensitive drum M has to be equal to time after the elapse of time Lkm/V obtained by dividing a distance Lkm from the contact point of the photosensitive drum K and the belt to a contact point of the photosensitive drum M and the belt by the conveying speed V of the belt. Similarly, time Tc when an image is irradiated onto the photosensitive drum C has to be equal to time after the elapse of time Lkc/V obtained by dividing a distance Lkc from the contact point of the photosensitive drum K and the belt to a contact point of the photosensitive drum C and the belt by the conveying speed V of the belt.
However, even if the irradiating time is accurately set as mentioned above, there is a case where a positional deviation (hereinafter, referred to as a color drift) occurs among the toner images of four colors of K, Y, M, and C due to an eccentricity or the like of each of the photosensitive drums. A technique for making the occurrence of the color drift inconspicuous by periodically making the generation timing of the color drift coincident has been disclosed (for example, refer to JP-A-2000-187428).
According to such a technique, a reference point is added to each of the four photosensitive drums and phase differences among the reference points of the four photosensitive drums are always held constant, thereby periodically making the generation timing of the color drift coincident. To always hold the phase differences among the reference points of the four photosensitive drums constant, a process for setting the phase differences among the reference points is executed in accordance with a predetermined procedure at a print start time point.
According to the process for setting the phase differences among the reference points, the belt is conveyed and while simultaneously rotating the four photosensitive drums, the reference points of the four photosensitive drums are detected by using reference mark detecting units. Subsequently, while the belt is conveyed, the photosensitive drums are individually stopped on the basis of each reference point detecting time (the photosensitive drum and the belt enter a rubbing state).
When the last photosensitive drum is stopped, the belt is also simultaneously stopped and the process for setting the phase differences among the reference points is completed. The stop time of each photosensitive drum is calculated from a relation between the reference point detecting time obtained by the reference point detecting units and rotational speeds of the four photosensitive drums. By setting the stopping timing as mentioned above, each time a printing step is executed, the phase differences among the four photosensitive drums are held constant, so that the color drift becomes inconspicuous.
A problem to be solved is that in the process for setting the phase differences among the reference points of the above conventional technique, while the belt is conveyed, the photosensitive drums are individually stopped on the basis of each reference point detecting time and simultaneously with the stop of the last photosensitive drum, the belt is also stopped. That is, according to such a process, the rubbing state between the photosensitive drum and the belt increases to a level larger than it is needed. Not only an abrasion between the photosensitive drum and the belt but also deflection occurs in the belt, so that the color drift is further liable to occur.